Among the conventionally-known rotary lawn mowers are ones which cut grass by rotating a cutter blade, accommodated in a downwardly-opening housing, along the grass on a lawn. In recent years, improvement of cutter blades of lawn mowers has been under way. One example of such lawn mowers is disclosed in Japanese Patent Application Laid-open No. 2006-67918 (hereinafter referred to as “the relevant patent literature”).
The lawn mower disclosed in the relevant patent literature includes a downwardly-opening housing, a rotation shaft extending in a vertical (up-down) direction in the housing, and upper and lower cutter blades accommodated within the housing for rotation about the rotation shaft. The upper cutter blade has a blade section formed along the front edge, in the rotating direction of the upper cutter blade, for cutting grass on a lawn. The lower cutter blade has a blade section formed along the front edge, in the rotating direction, of the lower cutter blade, and left and right air lift sections extending from the rear edge, in the rotating direction, of the lower cutter blade while curving obliquely rearward and upward. Such a lawn mower equipped with the upper and lower cutter blades can secure good grass cutting performance.
Generally, noise (unwanted sound) would occur in lawn mowers as a cutter blade rotates. Sometimes, depending on the place where the lawn mower is used, more weight or emphasis is placed on noise suppression performance than on the grass cutting performance. As one conceivable approach, a cutter blade focusing on the grass cutting performance and another cutter blade focusing on the noise suppression performance may be prepared in advance so that switching is made as appropriate between use of the cutter blade focusing on the grass cutting performance and use of the cutter blade focusing on the noise suppression performance depending on the place where the lawn mower is used. However, because such an approach requires preparation of the two different types of cutter blades, not only the lawn mower would increase in cost, but also it would be bothersome to appropriately store the cutter blade that is not in use.